This invention relates to a process for preparing bentonite clays, which when dispersed in water produce unexpectedly high solution viscosities. Such characteristics are highly desirable in numerous commercial applications.
Bentonites are naturally occurring ores which are mined in various regions of the world. Since these materials are highly colloidal and readily swell in water to form thixotropic gels, they are well-known for use as viscosity builders. This result obtains because bentonites are platey-type clays having a micaceous sheet structure. Such clays therefore are self-suspending, swelling and gelatinizing when mixed with water. Because of these viscosity building characteristics, bentonites find major utility as viscosity enhancers or builders in such areas as drilling muds and fluids, concrete and mortar additives, foundry and molding sands, compacting agents for gravel and sand as well as cosmetics.
Most natural bentonites are found in nature to exist in the sodium and/or the calcium form. The performance of a calcium bentonite as a viscosity builder can often be enhanced by its conversion to the sodium form.
The prior art details attempts to enhance the viscosity building characteristics of bentonite clays by several approaches. For the most part these involve working (or shearing) of the crude bentonite ore. The sequence generally calls for a (1) working, e.g. milling; (2) drying; and/or (3) pulverization sequence.
In some instances the use of an alkali pre-treatment is described to "activate" the clays, prior to the milling or working step.
Hentz, U.S. Pat. No. 4,371,626 thus discloses that alkali "activation" is only required for high calcium bentonite clays. It is suggested that there is an ion exchange mechanism involved where the sodium ion from either NaOH or Na.sub.2 CO.sub.3 replaces the calcium ion. Hentz teaches that crude sodium bentonite can be upgraded with respect to its viscosity building characteristics, without alkali treatment, simply by (i) shearing the crude clay; (ii) drying the clay; and (iii) grinding/pulverizing the dried clay.
Alther, U.S. Pat. No. 4,242,140 describes a process for upgrading crude clays of the bentonite type by (i) adding 1-10% by weight of NaOH or Na.sub.2 CO.sub.3 to the crude clay, or adding it during compacting step; (ii) compacting the activator treated material; and (iii) grinding.
No drying is required by the Alther process, the compacted clay requires no further drying, and is ground and screened to the desired mesh size.
Alther subsequently reviewed the state-of-the-art with respect to bentonite activation in a review article. He summarizes it as follows in "Improvement in Drill Mud Proper of Low Grade Bentonite by Simultaneous Chemical Activation and Compacting", INTERCERAM, Vol. NR 5, 1982, p. 503: